There is considerable attention being given to the use of high brightness LED (HBLED) technology as a light source to replace traditional incandescent lamps. The catalyst for introduction of white LEDs, first as indicators, and later for viable commercial illumination sources, has primarily been due to development and refinement of blue-LED material-science processes, in conjunction with appropriate yellow-phosphor coatings for what is termed secondary emission. The science of secondary emission has been long understood by those skilled in lighting technology and such science has previously provided the basis for fluorescent and most other gaseous discharge lamps.
In such a process of secondary emission, monochromatic light, generated within a phosphor-coated LED chip, causes the phosphor to emit light of different wavelengths. This has resulted in white HBLEDs, with rating of up to a few watts and lumen outputs, depending on color temperature, exceeding 100 lumens per watt.
The mechanism is much like that in a gaseous discharge tube lamp where ultraviolet light excites the phosphor coating on the inside of an evacuated glass tube to create visible white light. Interestingly, many of the difficulties in refining the technology of white LEDs relate to the same issues experienced with gaseous discharge lamps in mastering phosphor composition and deposition processes to achieve consistency and desired performance.
The fundamentals of incandescent lamp design have changed little in the last 75 years. Similarly, the design and performance of fluorescent lamps have not changed substantially in the last 40 years. That is to say, both incandescent and fluorescent lamp processes are considered to be mature technologies, with very little gain in efficacy (i.e., lumens per watt) expected in the near future.
High brightness LED's, on the other hand, are experiencing some gain in efficacy each year as scientists refine techniques for light extraction from the chip and slowly master the composition and deposition of phosphors. When many of these factors are better understood in the future and efficacy is further improved (a projection accepted by most industry experts) the LED lamps will be far more easily accepted and many of the present challenges will be mitigated. Until that happens, however, there are compelling reasons to develop novel techniques to enhance what now exists so as to accelerate commercial viability.
Two factors are driving the substantial interest in white-emitting HBLEDs as a candidate to replace incandescent lamps in a large number of general illumination applications: longevity and energy conservation.
The typical white HBLED chip, generally rated from 0.25 to three watts, if used properly, is expected to have a useful operating life of over 50,000 hours, dramatically longer than the 750-2,000 hours of a typical incandescent lamp and much longer than the typical 6,000 hours of a compact fluorescent lamp. Readily available HBLEDs can exhibit efficacies of more than 100 lumens per watt, 6-10 times better than either a regular or quartz-halogen version of an incandescent lamp.
While there is significant saving in bulb replacement expense over a number of years, it is the saving in electricity costs which presents the most significant benefit. In conditions of near-continual operation, such as in restaurants, hotels, stores, museums, or other commercial installations, the electricity savings can provide a very favorable return on investment, even with relatively high purchase prices, in 18-24 months. The potential for rapid payback is generally much more evident than for other highly publicized “green” technologies” such as hybrid vehicles, wind turbines, solar power etc.
There is widespread acceptance that white-light LED sources are attractive as possible incandescent replacement lamps, especially in those types where the LED lamp is at its best, namely as reflector-type lamps such as PAR 30, PAR 38, or MR16. LEDs are by their nature directional light sources in that their light is emitted typically in a conical 120-150 degree beam angle, whereas an incandescent lamp tends to radiate in a near 360-degree spherical pattern and needs loss-inducing reflectors to direct light. Compact fluorescent lamps, because they are very difficult to collimate, are very inefficient when used as directional light sources.
The LED lamp starts out in a better position in spot or flood lamp applications because of its inherent directionality. In fixtures for ceiling downlighting, outside security, or retail merchandise highlighting, the need is for directional lighting, a factor taking advantage of the LED lamp's inherent emission characteristics. Those with a reasonable knowledge of physics know that a point source of light is best for use with a reflector or collimator. A CFL, being the virtual opposite of a point source, is poor in this respect. An incandescent filament is much smaller but still needs a good-sized reflector. An LED chip, being typically no larger than a millimeter on a side, lends itself to many more options with much smaller reflectors and collimating lenses.
Consequently, while white HBLEDs may alone, or as a partner with the compact fluorescent lamp (CFL), replace incandescent filament lamps, it is in the reflector lamps where the performance and economics of white LEDs appear likely to have the more immediate impact. While the CFL has become widely commercialized, the LED lamp does have certain advantages, which over the long term could give it a substantial marketing edge. Specifically, compared to a CFL, the LED lamp is a) more compatible with standard lamp dimming methodologies b) can more easily operate in low temperature, c) has no mercury content d) retains its efficacy when dimmed e) is essentially immune to shock and vibration and f) is immune to the degradation which CFL's experience with repetitive on/off cycling.
Referring to FIG. 4, prior art LED sources generally emit light out of an opening opposite their bases, which can be equipped with a faceplate. When mounted vertically in a ceiling fixture, this arrangement allows light to shine from the LEDs in all directions below the source (180 degrees). This illumination pattern is similar to that of existing incandescent flood and spot lamps and has therefore been considered a desirable arrangement in LED sources that are intended to replace these types of lamps.